The invention relates to a self-regulating heating element which comprises as a heat source at least one resistor body of a material having a positive temperature coefficient of electrical resistance (hereinafter termed PTC resistor).
U.S. Pat. No. 4,147,927 describes such a heating element which is characterized in that the resistor body or resistor bodies is or are situated between metal bodies which, on the side facing away from the resistor body or resistor bodies, contact the inner surface of the casing in a heat-exchanging and abutting manner.
In accordance with the above-mentioned Patent Specification, the casing can be made of glass, ceramics or a heat-conducting elastic synthetic resin, if desired in a metal outer casing.
U.S. Pat. No. 4,104,509 describes a casing material which in practice best satisfies the requirements, said material consisting of a vulcanised synthetic resin material which is capable of resisting the highest operating temperature of the element, an electrically insulating, heat-conducting metal compound and, if desired, an additional filler material.
Preferably, the vulcanised synthetic resin material is silicone rubber. Magnesium oxide, trivalent iron oxide or aluminum oxide may be used as a heat conducting metal compound and silicon dioxide as a filler material.
For safety it is desired to surround the PTC resistors and the encasing metal bodies by a double insulation because the assembly is connected to the mains. Due to an error, for example, in the composition of the casing or the hardening depth, the casing of the elements may crack during use in which case it is very important that there is a second casing of a more elastic material which safeguards the user of the element from contacting the live metal parts.
Moulding the interengageable parts of the casing or encapsulating a moulded part with a second material are methods which in practice have too many drawbacks, and which cannot be automated.
A further possibility is to encapsulate the assembly of PTC resistor(s) and the encasing metal parts with an insulating synthetic resin foil, for example, of polyimide, before it is slid into the moulded casing. However, this method is complicated and costly too.